Apparatus for firing carbon-containing products

ABSTRACT

Method and apparatus for firing carbon-containing products which involve processing the products through four zones, including a heating zone, a zone for release and combustion of volatile materials, a final heating zone, and a cooling zone, and in which the final heating zone provides for the heating of the products from the inside out thereby to achieve uniform temperature distribution in this stage of the operation to provide a more efficient process and a more desirable product.

United States Patent [72] Inventors Paul Morel Le Vesinet; Jean-Pierre Givry, Ville DAvry; Philippe Voisin, Saint-Jean de Maurienne, Frame [21] Appl. No. 797,738

[22] Filed Feb. 10, 1969 [45] Patented May 1 l, 1971 [73] Assignee Compagnie Pechiney Park, France [32] Priority Feb. 19, 1968 [33] France [54] APPARATUS FOR FIRING CARBON-CONTAINING PRODUCTS 7 Claims, 2 Drawing Figs. [52] US. Cl 165/61, 165/120, 34/18, 13/7 [51] Int. Cl F25b 29/00 501 FieldoiSearch l65/1,2, 61,12,65,120;34/18; /7

[56] RefereneesCited 1 UNITED STATES PATENTS 2,621,218 12/1952 Juckniess 13/7 3,474,544 10/1969 Holden,Jr.etal

Primary Examiner-Charles Sukalo Attomey-McDougall, Hersh, Scott & Ladd ABSTRACT: Method and apparatus for firing carbon-containing products which involve processing the products through four zones, including a heating zone, a zone for release and combustion of volatile materials, a final heating zone, and a cooling zone, and in which the final heating zone provides for the heating of the products from the inside out thereby to achieve uniform temperature distribution in this stage of the operation to provide a more efficient process and a more desirable product.

PATENTED um I ran 3; 578,069

FIG.I

0 1 QB a FIG. 2

IN VENTORS PA u; Mona, JEA/V- Mae/: 5 Ewan BY PHILIP/95 Vow/v 772 0x944, 9% 46a Mia/d any/- APPARATUS FORv FIRING CARBON-CONTAINING PRODUCTS This invention relates to a method and apparatus for firing carbon-containing products. The method and apparatus which is designed for the manufacture of electrodes is also applicable to other carbon-containing products, such as refractory linings, crucibles, and graphitized products.

It is known to produce carbon-containing products from a paste formed of grains of coke or anthracite agglomerated with a binder, such as pitch, in which the agglomeration is carried out by molding under pressure, extrusion, or the like, and heated to a temperature which may be as high as 600 to l,200 C. while in a nonoxidizing atmosphere. The amount of carbon rises to a maximum upon elimination of the volatiles from the binder, which volatiles sometimes catch on fire during the operation, a phenomena which is referred to as the The firing operation is carried out in a chamber-type furnace having heated partitions, or in a tunnel furnace through which the parts travel on trucks, or in an electric furnace wherein the charge itself functions as the resistance. In order to avoid oxidation, the products to be fired are generally embedded in a reducing material, such as coke or anthracite, in particulate form. 4

In the chamber-type furnace, hot gases concentrate in the partitions. Since it is necessary that all of the parts in the furnace be heated in accordance with a predetermined time-temperature curve, it is important for the temperature throughout the partitions and furnace to be as uniform as possible.

The method of heat transmission varies during the firing cycle. While at low temperature, heat transmission is effected primarily by convection. As the temperature rises transmission of heat by radiation occurs in increasing amounts until radiation becomes the predominate means of transmission during the cycle when the product to be fired reaches maximum temperature. On the other hand, the hot gases give up heat all along their path of travel, since, at any given instant, the partition walls will be at a lower temperature because of their having been swept over by gases which have traveled through a longer path after entry into the furnace.

It is desirable, therefore, to make use of a stream of hot gases traveling at a high flow rate and a temperature increase which is as little as possible. The result is the circulation of a considerable amount of gas to the chimney at a temperature which is close to that at which the gas was introduced into the heating partitions. This results in excessive heat loss during the operation.

In a continuous operation with a chamber-type furnace, the firing cycle is shifted from one chamber to another, with one chamber being removed from the cycle for purposes of loading and unloading, the admission of fresh air occurring at the inlet of the following chamber while the evacuation of burnt gases is effected from the outlet of the preceding chamber. Proceeding from the chamber removed from the cycle, the first group of chambers is undergoing cooling with the fresh air introduced being progressively heated, in the next group of chambers equipped with burners, the baking is being completed, and with the last group of chambers being heated by the combustion gases from the chambers upon completion of the firing. The gases issuing from the latter chambers are exhausted to the chimney from the outlet of the chamber preceding the one removed from the cycle. When a cycle has been completed, the chamber which has been unloaded and loaded is reintroduced into the cycle for heating while the immediately preceding chamber, undergoing cooling, is brought into position for removal from the cycle for the discharging and charging operations.

The described arrangement of furnaces lacks flexibility in that any attempt to modify the temperature in any one chamber affects all of the others. The fires" are difficult to control with the result that such furnace arrangement is difficult to use for firing crude products having a high pitch content for graphetization. Furthermore, the cycles are long, such as about 20 days, and, in addition, the products that are produced have very high porosity and excessive amounts of fissures.

In tunnel furnaces formed with tunnels of refractory material, the combustion air is passed therethrough in countercurrent flow with the products to be treated, the latter of which are loaded on trucks of refractory material and embedded in granulated coke or anthracite. Burners are located in the central region of the furnace. Thus the products first engage the hot gases being discharged from the furnace before passing in front of the burners during which they are heated to maximum temperature and after which they travel through a cooling zone in heat-exchange relation with the fresh gases entering the furnace. The radiation from the walls of the furnace obviously contribute to the heat exchange.

Such furnaces have disadvantages which arise from the presence of large amounts of inert refractory material such as the trucks and filling material which consume heat and which retard heat transmission thereby to limit the rate of the rise and fall of temperature. Further, at operating temperature of l,30 O to l,350 C., the refractory walls and trucks have relatively short life and require frequent replacement. In addition, the cost for loading and unloading of the trucks is excessive.

In the resistance-type electric furnace, the products to be fired or baked, and embedded in a suitable filler material, are stacked between the current supply heads. The filling material not only protects the product from air oxidation and insulates the products from heat, but it provides resistance at the start while the product to be treated is electrically insulating and becomes conductive at temperatures of about 650 C.

Such furnaces are subject to serious disadvantages due to the difficulty of obtaining homogeneous distribution of product and filling material. This results in nonuniformity in electrical current with corresponding heterogeneous heat generation and temperature distribution. This effect is aggravated by the fact that the electrical conductivity of the product changes quickly from a very low value to a high value as the temperature passes through the range of 6000 to 650 C., thereby further to increase the concentration of current in superheated portions. Efforts have been made to obviate this disadvantage by limiting current density to permit heat conductivity to take effect for leveling out the temperature. The result is a longer firing period and decreased yield without material reduction in variation and quality of product secured. Again, a large mass of filling material is used with corresponding heat loss and increased costs for handling.

It is an object of this invention to provide a method and apparatus for firing or baking carbon-containing products in which the operation can be carried out in a short operating cycle, in which products of uniform quality can be obtained, and in which the products are relatively free of fissures.

These and other objects and advantages of this invention will hereinafter appear, and for purposes of illustration, but not of limitation, an embodiment of the invention in shown in the accompanying drawing, in which:

FIG. 1 is a diagrammatic view of an installation for carrying out the process of this invention; and

FIG. 2 is a diagrammatic sectional view of a tunnel furnace representing the first element in the installation of FIG. 1.

Briefly described, the concept of this invention resides in the treatment of the product to be baked in the presence of a gas which is nonreactive to the carbon-containing product, with the treatment being subdivided into four phases, namely, a heating phase, a phase for release and combustion of volatiles, and, finally, a heating phase and a cooling phase.

The installation employed in the practice of this invention comprises a tunnel furnace having a cooling means and which is subdivided into two successive zones which provide the first two of the phases with the gas and the products to be fired being passed therethrough in countercurrent flow. The tunnel furnace is followed by an airtight chamber equipped with electrical heating means for the generation of heat from within the interior of the product to be fired, such as by induction heating, with clips applied to the product for the passage of electrical current therethrough. Pipes are provided at the junction between the zones of the tunnel furnace for connection to exhaust fans for exhausting the gases to a chimney and other pipes are provided toward the free end of the zone for the introduction of fresh air.

The carbon-containing products are obtained by firing a crude product formed of a mixture of coke and binder, and extruded or molded either under high pressure, such as up to 350 bars, or by vibration, or combinations thereof.

The product is characterized by internal stresses. Although the product experiences an expansion due to the expansion of the occluded air on emission from the extrusion die or release from a mold, mechanical stresses exist due to the pressure of the occluded air and the pressure gradient through the mass.

During heating of the crude product, the binder undergoes a thermal evolution which causes it to pass through the conventional phases hereinafter referred to, the approximate limiting temperatures of each phase corresponding to a coil tar.

in a softening phase, covering the range up to 100 C., the binder is progressively softened without appreciable dimensional change.

In a nonevolutionary plastic phase, covering the range of 100 to 250 C., the softened binder becomes increasingly more fluid but without the release of volatile materials. The product becomes completely plastic and permits residual stresses, resulting from the shaping operations, to be released. in this phase, it is possible for cracking to occur along the fragile surfaces established during the extrusion or molding by a detachment of the stratified zones due to the pressure exerted by the occluded gases in the impermeable product.

In a swelling phase, covering the range of 250 to 450 C., the main portion of the volatile materials are released, accompanied by an expansion of the plastic mass of the product. This expansion is not accompanied by the formation of fissures or cracks because of the plasticity of the product. It is during this phase that the future macroporosity of the baked product is established.

In a plastic contraction phase, covering the range of 450 to 480 C., the amount of volatiles released decreases and a contraction or shrinkage occurs, accompanied by a decrease in the means diameter of the pores.

In a resolidification phase, covering the range of 480 to 500 C., resolidification of the binder occurs very suddenly. For any given material, the resolidification temperature depends somewhat on the speed at which the temperature rises. The temperature limits which are indicated correspond to increase in temperature at a rate greater than C. per hour.

Finally, a solid phase, covering the range above 500 C., corresponds to the further treatment of the product which undergoes an evolution of its mechanical characteristics. These latter, which are slight at the start, reach their maximum at a temperature which, for pitch, is in the region of 750 C., after which the mechanical characteristics remain substantially constant. The dangers of forming cracks in the product decrease as the temperature increases. They are practically nonexistent when the mass is heated without their being established over a wide temperature gradient.

In the case of industrial baking or firing, the product to be baked can have a considerable volume. The crude product, however, does have a low thermal conductivity so that, if the heating takes place from the outside, a temperature gradient will exist through the cross section such that several baking phases may be present simultaneously in a single product.

For carrying out the operations in a preferred manner, it is desirable to provide for a very slow rise in temperature until the residual stresses from the shaping operation have been released. It is possible, however, for this rise in temperature to be accelerated if the product is placed in the furnace immediately after it has been shaped, without intermediate cooling. After the release of such residual stresses, the rise in temperature can be increased to a very high rate, the permeability of the outer portions permit the release of volatiles without danger of altering the product. Finally, a temperature level should be maintained before progressing to the electric baking phase, in order to homogenize the product.

During the solid phase, the product can withstand fairly high stresses. A critical zone still exists at a temperature corresponding to the coefficient of maximum contraction, i.e., about 750 C. in the case of pitch. These is a danger of cracking on the periphery due to a differential contraction between the periphery and the interior.

Applicants obviate this danger by carrying out the heating, during the solid phase, within the actual mass of the product. In this way, any temperature gradient between the interior and the exterior is substantially eliminated. Apart from the elimination of cracks due to this gradient, this makes it possible, in the other phases, to use gases which are brought to a moderate temperature. From then onward, it is possible to provide circulation of the gas in a closed circuit with the gas receiving heat simultaneously from the combustion of the volatile vapors and from burners which are installed in the furnace. Thus this gas can be made nonreactive or even reducing, thereby to insure the protection of the product against oxidation and eliminate the necessity of enclosing the product in a protective layer, as in present practices. Furthermore, use is made of hot gas at a much lower temperature than employed in current furnaces, the maximum being of the order of 700 to 750 C. instead of 1 ,300 C. The result is a furnace of simple construction, much better refractory life, and reduced maintenance costs.

Referring now to the drawing, the installation shown in H6. 1 comprises a tunnel furnace subdivided into two zones 1 and 2. in the first zone I, referred to as the heating zone, the products pass from the temperature I, to the temperature t and the gases passed from the temperature T to the temperature T,. in the second zone 2, referred to as the zone for release and combustion of the volatile materials, the products pass from the temperature 2 to the temperature i and the gases from the temperature T to the temperature T The installation includes an enclosed chamber 30 provided with electrical heating means 32 and in which the products are brought from the temperature t to the temperature t.,, and a coding chamber 40 in which the products are cooled from a temperature to a temperature t The tunnel furnace 10 receives fresh air through its outlet 13. The air is immediately heated by the product undergoing baking and by the burners 21 to the temperature T;,. At the outlet end portion of the tunnel furnace, a zone is thus established in which the temperature 1 of the product undergoing baking remains substantially constant. This enables the product to be homogenized before it enters the chamber 30. The gases which exhaust from the tunnel furnace at the inlet end, at a temperature T,, are directed in part through line 14 to a chimney ll and returned in part through line 15 to the furnace for the introduction through pipes 12 in regions situated on either side of the junction between the zones 1 and 2.

The chamber 30 is equipped with internal electrical heating means, such as members 31 applied to the products to be heated, or an induction heating apparatus.

Cooling in chamber 40 can be effected by a water spray or by circulating an inert cool gas therethrough.

The travel of the products to be baked through the tunnel furnace can be effected by any known or conventional means, such as a continuous belt, trucks operating on a roller track and thrust means, or any other equivalent means capable of withstanding the temperature of the furnace.

The determination of the conditions used in the installation shown in FIG. 1 is obtained by the known methods. The preheating zone 1 operates as an undercurrent-heat exchanger, while the combustion zone 2 operates as a reactor in which the energysource is composed partly of the heat contributed by the combustion of the volatile materials released from the product and bumed with the supply of air 12 and partly by the heat contributed by the burners 21. The hydrogen takes up oxygen upon combustion to yield steam, and the carbon takes up oxygen upon combustion to yield carbon monoxide thereby to provide nonreactive or a reducing hot gaseous atmosphere. The presence of certain amounts of carbon dioxide does not present any inconvenience, especially in view of the inhibiting effect of the carbon monoxide.

The electrical heating chamber 30 insures a uniform rise in temperature through the cross section of the products up to about 1,200" C. in about 20 minutes. For this purpose, a current density of at least amperes per square centimeter is employed, in the case of heating by the Joule effect. For products of complicated shape or small dimension, heating by induction is to be preferred.

The products travel through the tunnel furnace 10 in the direction of the arrow 8 from the preheating zone 1 toward the combustion zone 2. From the outlet of the tunnel furnace, the products are introduced into the chamber 30 representing the electrical heating zone 3, and then into the cooling arrangement 40 which represents the cooling zone 4. The hot gases circulate in the opposite direction through the tunnel furnace, as indicated by the arrow 9. The tunnel fumace diagrammatically illustrated in FIG. 2 is identical with that of FIG. 1, but the circuit through which the gases travel outside the furnace 10 has been modified. The gases exhausted from the furnace at the temperature T pass first to a fan 13, from which an amount of gas equal to the amount added by the combustion of the volatile products in zone 2 plus the amount of fresh air admitted to the inlet of the furnace, is directed to the chimney l1. Excess gases are directed into the region on either side of the junction between zones v1 and 2, preferably after being reheated to a temperature of T in apreheater 14. In certain instances, reheating is unnecessary or might even be replaced by cooling. Valves 1S permit control of the distribution of the gases reintroduced into the furnace.

By way of example, reference is made to an installation comprising a tunnel furnace 10 having a width of 1 meter, a height of 0.60 meters, and a length of meters. Seven meters of the length serve for the preheating zone and the remaining 8 meters serve as zone 2 in which combustion and homogenization of the temperatures takes place. The electric heating chamber 30 has a length of 0.50 meters and the cooling arrangement 40 is formed of a fluidtight tank having a length of 0.5 meters and is provided with a water-sprinkling device.

The crude products introduced are formed of a mixture of petroleum coke with 1-7 percent coal tar, molded under a pressure of 325 bars. I

The temperatures recorded are as follows:

t C. (ambient temperature) ..C- T 150 t=300 C C T,=400 ta=675 C C T =720 t.= 1,200 C. t 400 C.

The described furnace is intended for firing or baking three electrodes per hour in which the electrodes have a dimension of 40 83 50 centimeters, representing a total mass of 695 kg.

By increasing the supply of gas, it is possible to reduce the length of the tunnel furnace.

The baked products which are obtained by the method of this invention and by the apparatus described are believed to constitute new and improved products which also represent an object of the invention.

It will be understood that changes may be made in the details of construction, arrangement, and operation without departing from the spirit of the invention, especially as defined in the following claims.

We claim:

1. An installation for the firing or baking of carbon-containing products through four phases comprising heating, release and combustion of the volatile materials, final heating and cooling, with the heating taking place in a nonreactive atmo'sphere, said installation comprising a tunnel furnace having an outlet end and an inlet end and in which the tunnel furnace is divided into two zones corresponding to the first two of the phases, means for passage of the products and a nonreactive gas through the tunnel furnace in opposite directions, a fluidtight chamber provided with electrical means for releasing heat from the interior of the products to be fired, including members applied to the product for causing an electrical current to pass therein, and a cooling chamber after the electrical heating chamber, said products passing from the outlet of the tunnel furnace to the heating chamber and then to the cooling chamber.

2. An installation as claimed in claim 1 which includes means for introducing air into the tunnel furnace through the outlet end and exhausting the combustion gases from the furnace at the inlet end and means for introducing products to be tired into the furnace at the inlet end and for removal from the outlet end.

3. An installation as claimed in claim 2 which includes inlet means adjacent the pump between the wnes of the tunnel furnace, and means for recirculating a part of the gas exhausted from the inlet end of the furnace to said inlet means for the recirculation of combustion gases through a portion of the tunnel furnace.

4. An installation as claimed in claim 3 which includes means for diverting a part of the gas exhausted from the inlet end of the tunnel furnace to a chimney.

5. An installation as claimed in claim 1 which includes burners in the tunnel furnace adjacent the outlet end portion for combustion of air introduced into the furnace to heat the products and to inert the gas.

6. An installation as claimed in claim 1 in which the electrical heating means for heating the products in the fluidtight chamber comprises an induction heating means.

7. An installation as claimed in claim 1 which is characterized in that the tunnel furnace includes a homogenization zone at constant temperature in its outlet end portion. 

2. An installation as claimed in claim 1 which includes means for introducing air into the tunnel furnace through the outlet end and exhausting the combustion gases from the furnace at the inlet end and means for introducing products to be fired into the furnace at the inlet end and for removal from the outlet end.
 3. An installation as claimed in claim 2 which includes inlet means adjacent the pump between the zones of the tunnel furnace, and means for recirculating a part of the gas exhausted from the inlet end of the furnace to said inlet means for the recirculation of combustion gases through a portion of the tunnel furnace.
 4. An installation as claimed in claim 3 which includes means for diverting a part of the gas exhausted from the inlet end of the tunnel furnace to a chimney.
 5. An installation as claimed in claim 1 which includes burners in the tunnel furnace adjacent the outlet end portion for combustion of air introduced into the furnace to heat the products and tO inert the gas.
 6. An installation as claimed in claim 1 in which the electrical heating means for heating the products in the fluidtight chamber comprises an induction heating means.
 7. An installation as claimed in claim 1 which is characterized in that the tunnel furnace includes a homogenization zone at constant temperature in its outlet end portion. 